SARS-CoV-2 seroprevalence in French 9-year-old children and their parents after the first lockdown in 2020

Introduction Children have been significantly less affected by COVID-19 than adults and presented with milder and less symptomatic forms of the disease. However, there has been suggestion that children older than 10 years and adolescents exhibits features closer to that of young adults. Most studies combine children in different age-groups and lack sufficient numbers to explore in detail age specificities. We report data on a population-based sample of 2,555 children at the pivotal age of 9 years. Methods In April 2020, the participants in two French nationwide cohorts of children, Elfe and Epipage2, were invited to take part into an online survey about Covid related symptoms and family life during the lockdown. A second questionnaire was sent on May 5. This questionnaire also proposed to the child included in the cohort and to one of his/her parents to take part into a capillary blood collection for Covid serology. Families who agreed to the serological survey were sent kits for dried blood spots self-sampling (DBS) with instructions. Samples were processed with a commercial Elisa test (Euroimmun®, Lübeck, Germany) to detect anti-SARS-CoV-2 antibodies (IgG) directed against the S1 domain of the spike protein of the virus. Results Children's acceptance rate for the serological survey was around 60%. 2,555 serological results were analyzed. The weighted prevalence of a positive Elisa Spike serology was 2.8% in 9 yr-old children (95% CI: 1.7%–4.0%). Positive serology was found in 8.6% (7.4%–9.7%) of parents who provided blood. There was a significant association (p < 0.001) between serology of the child and parent from the same household with an odds ratio of 13.8 (7.9–24.2). Discussion We have shown that 9-yr old children had a lower susceptibility to SARS-Cov2 infection than adults with the initial Chinese strain, similar to younger children and estimated that around 3% of them have developed antibodies against SARS-Cov2 in France after the first wave of the Covid-19 epidemics.

coverage coefficient, which takes into account that some eligible mothers were not contacted (number of eligible infants/number of infants included in the survey), was applied.

𝑊𝑒𝑖𝑔ℎ𝑡𝑚𝑎𝑡𝑒𝑟𝑛𝑖𝑡𝑦=𝑃1×𝑃2×𝑃3 (_1_)
For our study population, the fitted weight for each child was calculated by multiplying the maternity weight for each child with an adjustment coefficient for the non-response of the child to the SAPRIS serology survey (equation 2).This adjustment coefficient was calculated from estimated probabilities of response obtained by logistic regression including variables common to respondents and non-respondents (birth preparation sessions, father's employment at birth, father's age, mother's marital status at birth, alcohol consumption during pregnancy, twinship, mother's employment at birth).These probabilities were ordered to obtain sorted scores used to constitute homogeneous response groups in which the nonresponse was considered to be random.Thus, the maternity weight of respondents with a low probability of responding and uncommon characteristics was increased.
Finally, a calibration on margins was performed on the study population weights by using auxiliary variables (mother's age, region, life in couple, migration status, level of education and primiparity) obtained from the 2011 state register's statistical data and the 2010 French National Perinatal study and four variables from the closest available population at the time of the survey (2016) (type of dwelling, urban/rural city, region of current residence, type of household) for households with a child born in 2011.
Because of a specific attrition due to the low rate of respondents in our population study, the weights considered as extreme were truncated, which induced a bias but decreased the variance and the range of weights ( 22).

The weights reconstitute the population of children born after 33 weeks of gestation in metropolitan France in 2011
Epipage2 sample A statistical weight was calculated for each child surviving after initial neonatal hospitalisation according to the inclusion procedure.A P1 weight for each infant corrected for the sampling procedure (different duration of inclusion periods in different gestational age (GA) strata) by attribution of the following values 1, 35/26, 35/5 for respectively the 22-26 GA, 27-31 GA and 23-24GA strata.As the acceptation rate was high for the Epipage2 cohort, no correction for initial participation rate in the cohort was undertaken.Second a fitted weight for each child was calculated by multiplying the P1 weight for each child with a P2 adjustment coefficient for the response of the child to the SAPRIS serology survey (equation 2).This adjustment coefficient was calculated from estimated inverse probabilities of response obtained by logistic regression including the following variables common to respondents and nonrespondents (region of birth, gestational age strata, multiple pregnancy, maternal age strata, maternal country of birth (France yes or no), socio-professional class, partner at birth, educational level strata, primiparity, child sex, severe neonatal morbidity).Finally, a calibration on margins was performed on the study population weights by using the same variables used for P2.
The weights reconstitute the population of children born between 26 and 34 weeks of gestation weeks of gestation in France in 2011.

Total sample
The 37 children born between 33-34 weeks of gestation were excluded from the Elfe sample to obtain a sample disjoint from the Epipage 2 sample.The total population is simply estimated at the sum of the two samples